Jumper cables have come into common use for connecting a host battery to a weak battery for the purpose of starting an automobile or the like. It has long been known that there is a danger in making an improper connection. When such a connection is improper, with 12 volt batteries, there is the possibility of a 24 volt short circuit through the jumper cables. Often times due to age or corrosion, it is difficult to read the polarity marking on the terminals. Such a wrong connection may cause injury by reason of an exploded battery, or the acid electrolyte may squirt out of the battery vents. Further, such an improper hookup can damage the charging system of either the automobile containing the host or booster battery or the automobile containing the weak battery.
A number of attempts have been made in the prior art to provide an indication of an improper connection. Wolf U.S. Pat. No. 3,267,452 of 1966 employs a light bulb in one of the handles connecting clamps, and this light bulb is then used to determine whether there is a large volt difference in the circuit to be connected sufficient to light the bulb. The system has the disadvantage of requiring, in the first place, a socket connection separated from the handle for supporting the light bulb itself, and further requires, for the proper operation, that the bulb not be burned out. Also, a light bulb is particularly subject to failure by reason of rough handling or dropping of the cables, or by applying a voltage substantially exceeding the voltage rating of the bulb.
Another lamp type of polarity indicator is shown in Matheson U.S. Pat. No. 3,257,754 of 1966. Here, an electric light bulb is provided with its own separate leads to allow it to be removed and used as a trouble light and to be clamped to either battery.
In Godshalk U.S. Pat. No. 3,389,200 of 1968 there is shown a battery charger with a reverse polarity indicator. The reverse polarity indicator is in the form of an ammeter, and a fixed resistor is received within the handle of the battery charger clamp to limit the current through the ammeter in the event that the charger is being connected in a reverse state.